1. Field of the Invention
This invention relates to a process for diecasting graphite cast iron at a solid-liquid coexisting state.
2. Description of the Related Art
In general, cast iron is widely used in various fields such as automobile parts and the like because it is good in castability and can be cast into products of complicated shapes. To this end, if thin-walled products can be produced by industrially diecasting the cast iron, the weight reduction of the product can significantly be attained. However, the melting point of the cast iron is very high (not lower than 1150.degree. C.), so that there is no mold material durable to the melting temperature of the cast iron.
As the industrial diecasting process of the cast iron, it is possible only to conduct the diecasting at a temperature of solid-liquid coexisting state which is lower than the melting point of the cast iron and has less latent heat, so that it is strongly desired to industrially develop such a diecasting.
Although diecasting of cast iron is not yet industrialized, there is known a method of injecting a melt of the cast iron from a diecasting machine. When a melt of spheroidal graphite cast iron is diecast in the diecasting machine, there is a problem in the heat resistance of the mold as mentioned above, and also Ca or Mg as a graphite spheroidizing agent is easily evaporated at a molten state of the spheroidal graphite cast iron. In the latter case, even if the melt is formed in the vicinity of the diecasting machine as much as possible, counter measures should be taken to prevent the evaporation of the graphite spheroidizing agent or further adding the graphite spheroidizing agent to the melt.
In cases of conducting the diecasting at the solid-liquid coexisting state, there are known rheocasting process and thixocasting process. The rheocasting process is a process in which a slurry of semi-solidified metal composition is directly supplied to a diecasting machine and then injection molded therefrom, while the thixocasting process is a process in which a continuously cast billet or the like is reheated to a temperature of solid-liquid coexisting state and supplied to a diecasting machine and then injection molded therefrom. In the thixocasting process, the billet is reheated to a temperature lower than the melting point in a short time, so that there is caused substantially no evaporations of the graphite spheroidizing agent.
In the rheocasting process, however, the entrapment of air and inclusions is undesirably caused, and there are problems in the matching of throughput capacity between the continuous production device and the working device of the semi-solidified metal composition, the handling of the semi-solidified metal composition slurry, the process control and the like, so that this process is not yet industrialized.
In the thixocasting process, when the ingot of spheroidal graphite cast iron statically solidified is injected at the solid-liquid coexisting state, dendritic crystals entangle with each other to form a large lump. The lump moves through the diecasting machine, so that the crystals remain in the mold as a lump. As a result, only liquid phase metal is fed into the mold. Consequently a cast product having a uniform structure is not obtained.
As a measure for improving uniformization of the product structure, there is a method of using an ingot of cast iron having a granular primary crystal (in case of a hypo-eutectic structure, the primary crystal is ferrite). However, the ingots of granular structure for the diecasting are obtained by the following methods and have the following problems accompanied therewith.
1) A melt of the ingot is solidified with stirring. In this case, there is caused entrapment of air during the stirring, entrapment of broken pieces of the agitator, fluctuation of the composition and the like. PA1 2) A cast ingot statically solidified is subjected to plastic working to impart strain and then granulated by heating. However, it is difficult to adopt this method because the cast iron is poor in plastic workability. PA1 3) A melt of the ingot is added with an inoculating agent and then cast into a given shape. In this case, eutectic cells (crystal grain consisting of iron and graphite) can be made fine, but the effect of fineness of the primary crystal grains is small.